In-situ particle monitoring (ISPM) sensors can provide continuous monitoring of particulate contamination levels during key semiconductor process operations. Based upon light-scattering detection techniques, ISPM sensors are typically installed downstream of the process chamber, such as to a pump-line, and provide real-time measurement of variations in particle concentration and size during wafer processing. However, there are several inherent disadvantages to pump-line installation of a sensor apparatus. First, a particle depositing on a processed wafer cannot be measured with the sensor in the pump-line configuration. Second, and because ISPM sensors depend on various particle transport mechanisms to detect the particles generated upstream in the process chamber, ISPM sensor applications often produce poor correlation with the number of particles that deposit directly on the product wafer surface. In addition, the particle detection volume for ISPM sensors is also limited by the small cross-sectional area of the laser beam which is illuminating the particle (s).
To improve the correlation between ISPM sensors and the number of particles that deposit on the wafer surface, an advanced above wafer in-situ particle monitoring (hereinafter AWISPM) sensor has been developed. As described in U.S. Pat. No. 5,943,130, the entire contents of which are herein incorporated by reference, an AWISPM sensor is capable of monitoring particulate contamination levels within the process chamber. The sensor incorporates a scanned laser beam to create a large detection volume compared to stationary laser beam systems previously known to ISPM sensor technologies. Since the detection volume is only approximately 4 mm above the wafer surface, the capture rate of the AWISPM sensor is enhanced for particles that will deposit directly upon the wafer surface. The capture rate for particulate contamination is also improved by the larger detection volume, which provides a significant increase in the volumetric sampling rate. The AWISPM sensor provides information regarding the actual count, size, and velocity (speed) of the detected particles.
Because a scanned laser beam is used to detect the presence of the particle (s), a particle will be detected multiple times as it passes through the measurement volume, if the particle drift velocity is small compared with the laser beam velocity at the sample volume. Laser scanning is accomplished using either a resonant scanner or a rotating polygonal mirror. Either optical device results in a precisely defined period (e.g., frequency) associated with the scanned beam.
This defined period forms the basis of the present invention, allowing the detection and isolation of particle-pulse-envelopes (PPE's) from the continuous stream of low level electronic and optical noise (stray light) that is typically included in the observed signal. The principle contribution of the signal-processing algorithm is a substantial decrease in the level of false alarm counts at the minimum particle size detectability limit. Since the data stream is analyzed in the time domain relative to the scan frequency, PPE detection at or near signal-to-noise ratios of one can be accomplished.